Optimod-AM 9400 V1.2 Operating Manual - Orban

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INTRODUCTION ORBAN MODEL 9400
Best Location for OPTIMOD-AM
The best location for OPTIMOD-AM is as close as possible to the transmitter so that
its output can be connected to the transmitter through a circuit path that introduces
the least possible change in the shape of OPTIMOD-AM’s carefully peak-limited
waveform. This connection could be short lengths of shielded cable (for transmitters
with analog inputs) or a direct AES3 connection (if the transmitter has a digital input
available). If this is impossible, the next best arrangement is to feed the 9400’s AES3
digital output through an all-digital, uncompressed path to the transmitter's exciter.
If the programming agency’s jurisdiction ends at the link connecting the audio facility
to the transmitter, a variety of problems can occur downstream. (The link might
be telephone / post lines, analog microwave radio, or various types of digital paths.)
The link, the transmitter peak limiters, or the transmitter itself can all introduce artifacts
that a studio-located audio processor cannot control.
If the transmitter is not accessible:
All audio processing must be done at the studio and you must tolerate any damage
that occurs later. If an uncompressed AES3 digital link is available to the transmitter,
this is an excellent, accurate means of transmission. However, if the digital link employs
lossy compression, it will disturb peak levels by up to 4 dB. Lossy compression is
also inappropriate for another reason: it cannot accommodate pre-emphasized audio
(like OPTIMOD-AM‘s output) without introducing serious artifacts.
Unlike FM, where the transmitter usually can be set up to provide preemphasis,
AM transmitters are universally “flat.” Therefore, unlike FM,
there is no option when using lossy compression to de-emphasize at the
output of OPTIMOD-AM and then to restore the pre-emphasis at the
transmitter. The best one can do is to use NRSC pre-emphasis, apply NRSC
de-emphasis before the lossy link’s input, and then re-apply NRSC preemphasis
at the link’s output.
If only an audio link is available, use the 9400’s left and right audio outputs and feed
the audio directly into the link. If possible, request that any transmitter protection
limiters be adjusted for minimum possible action — OPTIMOD-AM does most of that
work. Transmitter protection limiters should respond only to signals caused by faults
or by spurious peaks introduced by imperfections in the link. To ensure maximum
quality, all equipment in the signal path after the studio should be carefully aligned
and qualified to meet the appropriate standards for bandwidth, distortion, group
delay and gain stability, and such equipment should be re-qualified at reasonable
intervals. (See Optimal Control of Peak Modulation Levels on page 1-8).
If the transmitter is accessible:
You can achieve the most accurate control of modulation peaks by locating
OPTIMOD-AM at the transmitter site or by connecting it to the transmitter through
an uncompressed digital STL.
Because OPTIMOD-AM controls peaks, it is irrelevant whether the audio link feeding
OPTIMOD-AM’s input terminals is phase-linear. However, the link should have low
noise, the flattest possible frequency response from 30-9,500, and low nonlinear distortion.